Enterohaemorrhagic Escherichia coli O157:H7 (EHEC O157) is a member of the attaching and effacing Escherichia coli (AEEC) (3) that form specific structures known as attaching and effacing (AE) lesions in the host intestinal epithelial wall, which allow EHEC O157 to intimately attach to the epithelial membrane in order to achieve colonization (18, 22, 24). In AE lesion formation initial attachment of the bacterium is followed by the injection of bacterial proteins into the host cell (8, 17, 21) through a specialized translocation apparatus, termed a type III secretion system (TTSS). This results in the cytoskeletal rearrangement and effacement of the microvilli. Finally a 94-kDa bacterial outer membrane protein, termed intimin is required (19), resulting in the formation of the bacterium-host cell pedestal structure (9, 10, 27, 37).
A number of enteric bacteria, including EHEC, are known to produce and/or respond to chemical signals called autoinducers. The use of this cell-to-cell signaling mechanism facilitates enteric microbes to regulate important traits that allow them to successfully colonize and/or start infection in their host (20). EHEC virulence-specific genes are regulated by quorum sensing (QS) (34, 35) mediated by the autoinducer-3/epinephrine/norepinephrine signaling system (36). Autoinducer-3 (AI-3) is a molecule produced by the commensal gastrointestinal microbiota that seems to resemble the hormones epinephrine and norepinephrine produced by the host (36) and is believed to allow the enteric pathogens to organize a concerted activation/repression of specifically required genes. Furthermore, an EHEC sensor kinase, QseC, which binds AI-3 and the hormones epinephrine/norepinephrine and regulates virulence in a rabbit infection model provides evidence that this QS system participates in interkingdom cross-communication (5). Thus, enteric pathogens possess an extremely complex regulatory system that is used to systematically compete in such a challenging environment and inhibition of this QS system may lead to an attenuation of virulence.
Salmonella spp. are widespread with in the environment. S. typhimurium DT104 typically is resistant to the antibiotics ampicillin, chloramphenicol, streptomycin, sulphonamides and tetracycline (R-type ACSSuT) (48). Salmonella enterica serovar typhimurium requires the expression of the TTSS for a number of important virulence factors like bacterial invasion, macrophage apoptosis and enteropathogenesis (41, 43, 44, 46 and 47). TTSS gene transcription is activated in response to environmental signals (39, 40 and 45). Cattle are thought to be a primary reservoir through which Salmonella multi-resistant pathogens can enter the food supply.
The human gastrointestinal tract harbors a complex microbial ecosystem containing a large number and variety of bacteria that has a major impact on gastrointestinal function and thereby on human health and well-being. Among these, some opportunistic bacteria are considered to be detrimental and cause adverse conditions such as diarrhea, infections, gastroenteritis and endotoxaemia, while other bacteria are considered “probiotic”, in that they perform beneficial functions for the human organism (49).
Probiotic bacteria are known to stimulate the immune system and exert a competitive exclusion of pathogenic and putrefactive bacteria, reduce the amounts of ammonia and cholesterol in the blood, and promote absorption of minerals (50). Additionally, probiotic bacteria produce antagonist effects against pathogenic microorganisms; stimulate the immune system; improve lactose digestion; are lipolytic, thereby allowing fats to be more digestible; reduce plasma cholesterol; protect the intestinal mucosa, thereby assuring effective assimilation of the nutritive substances; produce polysaccharides that are active on some tumors; and reduce viability of some enzyme-producing microorganisms which catalyze the conversion of procarcinogenic substances into carcinogenic substances. It is believed that the probiotic bacteria exert their effects in a synergistic manner to curtail and retard the growth of pathogenic and detrimental bacteria of the gut (51 and 52).
It is believed that the health and well being of people and animals can be positively or negatively influenced by the microorganisms which inhabit the gastrointestinal tract, and in particular the large bowel. These microorganisms through the production of toxins, metabolic by-products, short chain fatty acids, and the like affect the physiological condition of the host and improve the physiological well being of the host. As a result, research has focused on using probiotic cultures in a variety of compositions and methods to improve health.
For example, US 20040161422 discloses a nutritional food product comprising at least one probiotic bacteria to improve gut function. U.S. 20040115177 discloses methods of administering probiotic bacteria to livestock animals in an amount effective to reduce the amount of hazardous bacteria. Dietary supplements such as those for example sold as part of the PARINAT™ line is formulated with Lactobacillus acidophilus strain L.B. and is stated to be beneficial for general digestive and intestinal problems.
Studies have also determined that L. acidophilus La-5 may affect virulence-related gene expression in Escherichia coli O157:H7 (29). La-5 cell spent medium was used and found to affect bacterial transcriptional regulators, however, the studies were all conducted in vitro or Escherichia coli cultures and thus the conclusions could not support or identify the bacterial factor(s) responsible for the regulation of the EHEC O157 QS system. It was thus concluded in the study that animal models were required to characterize the efficacy and potential use of the L. acidophilus La-5 in mammalian therapeutic embodiments.
In view of the foregoing, it would be desirous to isolate and characterize the factor(s) produced by probiotic bacteria that provide beneficial effects in mammals for prophylaxis, prevention and treatment of harmful bacterial infection as well as use of such molecules as nutritional and food supplements for general health.